1. Field of the Invention
The present invention relates to a multilayer ceramic substrate, a method for producing the multilayer ceramic substrate, and composite green sheets for forming the multilayer ceramic substrate. In particular, the present invention relates to a multilayer ceramic substrate produced by a zero-shrinkage process, a method for producing the multilayer ceramic substrate, and composite green sheets for forming the multilayer ceramic substrate.
2. Description of the Related Art
For example, Japanese Unexamined Patent Application Publication No. 2000-25157 (Patent Document 1) discloses a multilayer ceramic substrate that is related to the present invention. Patent Document 1 discloses a composite laminate and a method for producing the composite laminate by a zero-shrinkage process. More specifically, Patent Document 1 discloses the composite laminate which can be produced while shrinkage due to firing is inhibited and which can be used in an as-fired state, and a method for producing the composite laminate. As a preferred example, a multilayer ceramic substrate having a structure described below and a method for producing the multilayer ceramic substrate are disclosed.
The multilayer ceramic substrate includes a base layer made of an aggregate of a first powder including a first ceramic material and a glass material, and a constraining layer made of an aggregate of a second powder containing a second ceramic material that is not sintered at a temperature at which the glass material is melted. At least a portion of the first powder is in a sintered state. The second powder is in an unsintered state. Particles of the second powder are bonded to each other by diffusion or flow of a portion of the first powder including the glass material into the constraining layer.
To produce such a multilayer ceramic substrate, a green laminate including a green base layer including the first powder and green constraining layer including the second powder is formed. Then, the green laminate is fired. In the firing step, at least a portion of the second powder is sintered. Furthermore, in the firing step, a portion of the first powder, typically, a portion of the glass material included in the second powder diffuses or flows into the constraining layer. Thus, although the second powder is not sintered, the particles thereof are bonded to each other by a portion of the first powder, in particular, by the glass material.
According to the above-described production method, since the second powder is not sintered during the firing step, the constraining layer including the second powder has the effect of inhibiting the shrinkage of the base layer to inhibit the shrinkage of the entirety of the multilayer ceramic substrate due to firing, thereby reducing a variations in dimensions of the resulting multilayer ceramic substrate. Furthermore, in the resulting multilayer ceramic substrate, the particles of the second powder included in the constraining layer are bonded to each other by diffusion or flow of a portion of the second powder including the glass material, thus eliminating the need to remove the constraining layer later.
However, when the technique described in Patent Document 1 is utilized, disadvantageously, the glass material does not always sufficiently penetrate into the constraining layer, depending on the type of glass material included in the base layer. The main reason for the problem may be that when the glass material melts, the glass material has high viscosity and thus has low fluidity.
To solve this problem, a low-viscosity glass material is used. However, when the low-viscosity glass is selected, for example, when capacitance is formed between the base layers, the base layers have an excessively low dielectric constant. Thus, disadvantageously, target capacitance cannot always be obtained. In other words, selecting a glass material having low viscosity and satisfying electrical properties required and adjusting the composition of the glass material require a lot of time and are often technically difficulty.
On the other hand, the thickness of the constraining layer has been reduced in order to solve the problem of the insufficient penetration of the glass material into the constraining layer. However, at a reduced thickness of the constraining layer, the shrinkage-inhibiting effect of the constraining layer is degraded, thereby disadvantageously causing the occurrence of warpage of the resulting multilayer ceramic substrate.